As one of the key bonding parameters for thermosonic flip chip bonding, bonding pressure plays an important role for the bonding process. Moreover, as the load of the ultrasonic transducer system, bonding pressure affects directly the process of ultrasonic propagation, so it will influence the bonding quality. In this paper, in the condition that the bonding pressure was tuned regularly, the impedance characteristics of ultrasonic transducer were measured by a impedance analyzer, and the ultrasonic energy was measure by a laser Doppler vibrometer, and the shear bonding strengths at different bonding pressure settings were attained. It is shown that when the bonding pressure increases, the resonance frequency and resistance of transducer will increase, while the conductance and ultrasonic energy will turn down, and a stable and satisfied bondability can be found at a moderate pressure window. The impedance model for ultrasonic transducer was created by an equivalent circuit, and used to analyze the experimental effect of bonding pressure on ultrasonic propagation for transducer.
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